Hope this is handy to know for someone with a mechanical spring tensioner.
I just found by playing around with it that the mechanical tensioner off my 1999 m52tub25 can be regreased, and its now as good as new.
You basically need to remove the tensioner from the car, pop the dust cover off the bearing, and then using a torx unscrew the bolt which holds the pulley wheel onto the tensioner. Once the pulley wheel is removed, use a fine blade to pop the rear dust seal carefully which then exposes the ball bearings/race (if you can't see the balls, pop the other side). Inspect the bearing for wear - mine had around 150k on it and virtually no grease left, but was smooth spinning and no visible wear (no chips etc), it just very loose to spin and a little noisy. Side to side play was evident but not excessive and no worse than a new one i'd bought. Pack some high melting point grease (CV Joint etc) into it and reassemble. Now spins very smoothly and silent too - no discernible difference to the brand new Febi one that I ordered off c3bmw for 40 odd quid recently (which will be going on ebay).
Hope this helps someone save a few bob - easy to check before splashing out on a new one - i'll be regreasing the aircon one soon. Should be a 100k service item IMO.